This invention relates to a light beam scanning apparatus utilized in a picture image forming apparatus such as a laser printer, a digital copying machine or the like, and more particularly to an improvement of an f.theta. lens for use in a light beam scanning device.
A picture image forming apparatus generally comprises modulating means such as a ultrasonic wave light modulator which modulates a laser beam generated by a laser oscillator to form a picture signal, and a light beam scanning device comprising deflector means such as a rotary polygonal mirror which deflects the modulated laser beam at a constant angular speed, and a lens system adapted to focus, the deflected laser beam on a scanned surface of a photosensitive member.
In such a light beam focusing device, so-called f.theta. lens is widely used as the lens system. The f.theta. lens is used for the purpose of correcting the deflection of the laser beam deflected by deflecting means such as the rotary polygonal mirror so as to make equal the scanning speed of the spot of the focused laser beam. The term f.theta. lens is named by the fact that the laser beam incident to the lens from a deflection point at an angle of .theta. with respect to the optical axis of the lens is focused to a position f.times..theta. spaced from the optical axis the lens having a focal length of f.
Various types of the f.theta. lens have been proposed. For example, Japanese laid open patent application No. 137631/1978 discloses a type made of a combination of lenses having negative and positive powers, Japanese laid open patent application No. 172109/1986 discloses another type made of a combination of lenses having positive and positive powers, and Japanese published patent application No. 496884/1986 discloses still another type made of a single lens.
In the f.theta. lens comprising a combination of lenses having negative and positive powers as disclosed in Japanese laid open patent specification No. 137631/1978, since the negative power is large, the back focal distance is long with the result that the light path length between the f.theta. lens and the scanned surface becomes long. Thus, it is difficult to make the light beam scanning compact. On the other hand, in the f.theta. lens comprising a combination of two lenses of positive power as disclosed in Japanese laid open patent application Nos. 172109/1986 and 175607/1986, since the lens located closer to the deflection point, at which the beam is deflected, has a stronger power and since the two lenses are located at relatively remote positions from the deflection point, the sizes of two lenses become large. This also makes it difficult to make compact the light beam scanning device. Moreover, the f.theta. lens made of a single lens as disclosed in Japanese published patent application No. 48684/86 has a small range of deflection angle so that it is difficult to provide a wide picture angle. For this reason, a considerably long light path length is necessary to obtain a desired scanning range. This also makes it difficult to miniaturize the light beam scanning devices.